The in-plane magnetic recording mode, wherein magnetic anisotropy is exhibited along the in-plane direction of the discs, has been used for hard discs used for some time. Nowadays, however, a perpendicular magnetic recording mode is suggested to improve the linear recording density (recording density in the direction of tracks) of discs.
In hard discs in the perpendicular magnetic recording mode, information is recorded in their magnetic film having perpendicular magnetic anisotropy (for example, TbFeCo).
In the perpendicular magnetic recording mode, a soft magnetic film is formed as a backlayer between a substrate and a magnetic film in order to record and reproduce information in this magnetic film at a high density and a high efficiency (see Japanese Unexamined Patent Publication No. Hei 06(1994)-76202).
A backlayer is formed as a film having magnetization direction in the in-plane direction of a disc. This is a layer for obtaining a large and sharp magnetic field by magnetic interaction generated between a magnetic head and the backlayer so as to carry out perpendicular magnetic recording in the magnetic film with high efficiency. From the viewpoint of the efficiency of recording and reproducing, a magnetic material having a small coercive force Hc, such as FeNi based alloy, is used in the backlayer.
<Kinds of Noises>
In order to heighten the reliability of the quality of signals for recording and reproducing, it is desired to decrease noises caused by the backlayer and make the saturation magnetization Ms as large as possible. Noises caused by the backlayer are classified into the following three kinds:    (a) stripe-form magnetic domain noises, (b) spike noises, and (c) media noises based on random magnetic domains.(a) Stripe-Form Magnetic Domain Noises
Noises generated by an inclination of magnetization from the in-plane direction to the perpendicular direction in the case that the film thickness of the backlayer is made large (for example, 150 nm or more).
(b) Spike Noises Spk
Noises generated in portions of a disc where magnetic direction in a backlayer is uneven in the case that the film thickness of its backlayer is made large (for example, 70 nm or more). The noises are noises generated by the matter that the backlayer forms Bloch magnetic walls.
(c) Media Noises Nm Based on Random Magnetic Domains
Noises generated in a broad band from a low frequency band to a high frequency band since random and large magnetic domains are formed in the case that the backlayer exhibits no soft magnetic property.
In order to decrease such noises, some countermeasures are suggested. For example, stripe-form magnetic domain noises are decreased by making the backlayer into a laminated structure wherein some soft magnetic layers and nonmagnetic layers are laminated and setting the film thickness of the soft magnetic layers and that of the nonmagnetic layers to about 100 nm and about 5 nm, respectively.
By making the backlayer into a laminated structure as described above, setting the film thickness of its soft magnetic layers and that of its nonmagnetic layers to about 200 nm and about 5 nm, respectively, and repeating this period several times, the amplitude of spike noises spk are suppressed. In order to decrease spike noises spk, it is known that it is necessary to make easy axes of magnetization of the soft magnetic layers even along the radius direction of the disc substrate.
In order to decrease medium noises Nm and to cause the backlayer to exhibit soft magnetic property, it is generally necessary to make magnetic domains of the backlayer small.
<Countermeasures for Decreasing Noises>
It is generally known that: when the magnetic wall structure of the backlayer turns to Bloch magnetic walls, spike noises spk are easily generated; and when the magnetic wall structure turns to Neel magnetic walls, no spike noises spk are generated. It is also known that: as the film thickness of the backlayer is larger, the backlayer is further dominated by Bloch magnetic walls; and in the case that the film thickness is thin (about 20 nm), the layer is dominated by Neel magnetic walls.
(1) Thus, suggested is a magnetic recording medium wherein the film thickness of a laminated unit of a backlayer is made small to decrease spike noises, thereby changing its magnetic wall structure from Bloch magnetic walls to Neel magnetic walls and having a multi-layered structure comprising thin films composed of soft magnetic layers and nonmagnetic layers (Japanese Unexamined Patent Publication No. Sho 61(1986)-5423). Therein, however, the film thickness of each of the periods in the backlayer is about 100 nm or more; therefore, it cannot be said that spike noise spk are sufficiently decreased.
(2) There is suggested a soft magnetic backlayer wherein soft magnetic layers (FeCo films) and nonmagnetic separating layers (Cr layers) are alternately and repeatedly laminated in order to decrease noises based on magnetic fluxes leaking out from magnetic walls in the soft magnetic backlayer (Japanese Unexamined Patent Publication No. Hei 06(1994)-136542). However, the FeCo films and the Cr layers are each made into a film thickness of 1 μm, and are each very thick. It can be therefore considered that Bloch magnetic walls are formed in the FeCo films. Accordingly, even if the noises based on the leaking magnetic fluxes can be decreased, the amplitude of spike noises spk cannot be easily suppressed.
(3) A structure for decreasing medium noises in the case of using a FeAlSi multi-layered film as a soft magnetic layer is also suggested (IEEE Trans. Magn., vol. 37, p. 1586, No. 4, 2001, “Low Noise FeAlSi Soft Magnetic Under-Layer for CoPtCrO Double Layered Perpendicular Recording”).
Therein, a soft magnetic backlayer is made into a multi-layered structure wherein soft magnetic layers (FeAlSi layers) and nonmagnetic layers (C layers) are alternately laminated. In this structure, media noises can be made remarkably lower than in a structure having a FeAlSi mono-layered film. However, the saturation magnetization Ms of the backlayer is a low value of about 800 emu/cc or less. Thus, no sufficient recording efficiency can be given. Additionally, no mention is made of a decrease in spike noises spk.
About presently-used material having a large saturation magnetization Ms, it is difficult to cope with both of the matter that the material is softly magnetized and the matter that easy axes of magnetization of the material is made even along the radius direction of a medium substrate. It is therefore difficult to decrease spike noises spk.